This invention relates to phyllosilicates which have been modified to increase their dispersibility in polymers and methods for producing the same. More particularly, the present invention relates to mineral fillers which have been edge modified to improve their dispersibility in polymer matrices and methods for making composite materials using the mineral fillers.
Mineral fillers are used extensively to enhance the performance of a wide range of thermoplastic and thermosetting polymers. Physical properties which are improved by fillers include stiffness, strength, impact and temperature resistance, improved dimensional stability, surface hardness and scratch resistance. Other properties improved with fillers include improved chemical resistance, electrical resistance, and flame retardancy. Mineral fillers can also be used to reduce the thermal expansion coefficient of thermoplastics and permeability to gases and liquids. The most commonly used mineral fillers in plastics are calcium carbonate, wollastonite, silica, and the phyllosilicates such as kaolin, talc, and mica. Talc is unique in that its surface is naturally hydrophobic and therefore compatible with olefinic polymers. On the other hand, calcium carbonate, silica, wollastonite and the other phyllosilicates are hydrophilic and must be surface treated in order to improve their dispersion and interaction with the polymer matrix. The surface treatment of hydrophilic phyllosilicates includes reaction of the basal surface of the mineral with organosilanes, modified oligomers and polymers containing anhydride functional groups, and a wide variety of surfactants. Maximum improvement in mechanical and barrier properties of polymers occurs with the use of well-dispersed platy minerals possessing high aspect ratios. and small particle sizes. The aspect ratios of platy minerals such as mica, talc and kaolin are typically in the range of 30 to 100.
Since the late 1980""s the focus of much research around the world has shifted from the traditional mineral fillers to the incorporation of fully exfoliated smectite clays, primarily montmorillonite with its extremely high aspect ratio, into a variety of thermoset and thermoplastic polymers. The aspect ratios of exfoliated smectite clays can range from 100 to 1,000 or more. The exfoliation and nanoscale dispersion of small amounts of smectite clays into polymers leads to composite materials with enhanced physical features, but with significant reductions in weight as compared to traditional mineral-filled polymers. Like the other hydrophilic phyllosilicates, the smectite clays must be surface-treated to render them compatible with olefinic polymers. The approach that has been most often used is based on the technology utilized for the last fifty years to make organoclays as Theological control agents in paints, inks, greases, etc. This approach utilizes quaternary amine-based surfactants to render the basal surface of the clay compatible with the polymer matrix. Various high-molecular-weight quaternary ammonium salts have been used such as dimethyl dihydrogenated tallow ammonium chloride, dimethyl benzyl hydrogenated tallow ammonium chloride, and methyl benzyl dihydrogenated tallow ammonium chloride. Other onium ions that have been used include the phosphonium and sulfonium groups. Surprisingly, this approach has not been very successful in promoting clay exfoliation in olefinic polymers such as polyethylene and polypropylene and their copolymers.
In any organoclay application, and especially in the preparation of composite materials, obtaining a good dispersion of the clay has always been problematic. Smectite clays have extremely large surface areas and because of their nanoscale their behavior is dominated by a complex balance of surface chemical forces. It is well known to those skilled in the art that maximum organoclay dispersion in organic solvents requires the addition of low-molecular-weight polar organic compounds. Various xe2x80x9cpolar activatorsxe2x80x9d as they are called, have been recommended and include low-molecular-weight ketones and alcoholsxe2x80x94with methanol and acetone being preferred. The polar activators are typically combined with small amounts of water and are used at levels ranging from 20 to 60 weight percent relative to the weight of the organoclay. Propylene carbonate has been recommended where the volatility of the activator is a concern. It is believed that the polar organic compounds encourage delamination and dispersion of the organoclay by solvating the high-molecular-weight ammonium ion at the basal surface of the organoclay which in turn affects the inter-platelet associations (i.e., basal spacing) resulting from the van der Waals attractions between surfactant chains and the clay surface. In rheological applications, a small amount of water is added with the polar activator to promote gellation via hydrogen-bond bridging between hydrophilic platelet edges. To this end, full rheological effectiveness requires unobstructed access to the hydrogen bonding sites on the clay edges. In composite material applications, the organoclay designs traditionally left the platelet edges untreated with the belief that the edge contribution to the hydrophilic lipophilic balance (HLB) of the organoclay is insignificant.
Pioneering work in the 1940s showed that increasing chain length of the amine and increasing amine loading leads to more complete coverage of the basal clay surface. This work is discussed in J. W. Jordan, B. J. Hook, and C. M. Finlayson, J. Phys. Colloid Chem. 54, 1196-1208 (1950). For example, approximately 80 percent of the basal surface is covered by amine molecules lying flat at an octadecylamine loading of 100 milliequivalents per 100 g of clay. However, maximum solvation of the hydrocarbon chains of the amine would require the hydrocarbon chain to lift off from the clay surface thereby exposing a hydrophilic, silicate surface. Jordan postulated that the polar organic activators facilitated the solvation of the hydrocarbon chains by simultaneously lifting the hydrocarbon chains on end and shielding the exposed silicate surface.
Self-activating organoclays have been described in the patent literature and represent an improvement in performance. Self-activation has been achieved through various approaches including manufacturing and compositional modifications. For example, a common approach is to overtreat the clay with a 10 to 25 percent excess of the quaternary amine above the ion exchange capacity of the clay. To maximize the self-activating characteristic, this treatment approach usually requires that amine exchange of the clay be carried out in the presence of low-molecular-weight polar activators such as alcohols, ketones, ethers, carboxylic acids, carboxylic esters, and amides. In a slight variation on his approach, higher molecular weight anionic compounds such as carboxylic acids having low water solubility have been used as self-activating agents in conjunction with amine treatment. In this approach, the anionic carboxylic acid forms a water-insoluble complex with the ammonium ion which then attaches to the basal surface of the clay leaving the edge unobstructed.
Analogous approaches have been used to enhance the exfoliation of organoclays during the preparation of a variety of clay/polymer composite materials wherein a high-molecular-weight polar compound is used to activate the organoclay. Examples of activators which also function to compatibilize the organoclay with the polymer matrix include, polyolefin oligomers with telechelic OH groups and maleic anhydride-modified polyolefin oligomers. Oligomeric activators have been used at levels comparable to those of the low-molecular-weight polar activators, i.e., 30 to 100 weight percent relative to the weight of the organoclay. Because of the higher molecular weight of the oligomeric activators, the total organic loading on the organoclay necessary to achieve the desired degree of exfoliation exceeds 70 to 75 weight percent relative to the weight of the organoclay making this approach both expensive and inefficient. In addition, organic solvents are often required to facilitate intercalation of the oligomer which increases cost and manufacturing difficulty. Additionally, the efficiency with which the high-molecular-weight compatabilizers increase the basal spacing of the organoclay is surprisingly low. For example, telechelic polyolefins reportedly increase the basal spacing of an amine-treated montmorillonite from 33 xc3x85 to only 38 xc3x85 at a mixture ratio of 1:1. This small increase in basal spacing suggests that not all of the oligomer becomes intercalated within the organoclay gallery. Because of the polar functional groups employed by this approach, it is not unreasonable to presume that a portion of the oligomer attaches to the edge of the clay and may actually block access to the organoclay galleries.
Prior art has focused almost exclusively on the modification of the basal surfaces of the phyllosilicates. The reasonableness of this approach has been supported by the fact that the basal surface comprises over 95 percent of the total surface area of smectite clays. Hence, any affect that the edge surface area might have on dispersion behavior was believed to be inconsequential. However, phyllosilicates have a tendency to stack in ordered arrays called booklets. This stacking means that prior to exfoliation the contribution of the edge surface area to the overall surface area is not insignificant. Unfortunately, ways to address this issue have not been forthcoming in the literature. Additionally, under current methods, large quantities of volatile, low-molecular-weight, polar activators are required to ensure complete exfoliation of organoclays in nonpolar systems. In the formation of clay/polymer composite materials, the volatile, low-molecular-weight, polar activators are undesirable and are replaced by surface-active oligomers. However, the amount of oligomeric activator required is 30 to 100 weight percent, or more, relative to the weight of the organoclay making the approach impracticable.
One approach to designing organoclays that does not focus solely on basal surface modification can be found in U.S. Pat. No. 4,412,018. This patent describes the treatment of the clay edge with anionic polyacrylates to facilitate exfoliation in slightly polar polymers. While the approach of using a polymeric dispersant is claimed to enhance exfoliation in nylon, it is not well suited to yield an improvement in nonpolar polymers, such as polyolefins. This is because the use of polymeric dispersants makes it difficult to precisely control charge location and charge density, which directly influence the surface HLB value of organoclays.
Thus a need exists for a method of increasing the dispersibility of organoclays in polymer systems without the need for large quantities of traditional polar organic activators.
The present invention provides a method for producing edge modified phyllosilicates which includes the steps of adsorbing organic surfactants, such as organophosphorous compounds and/or organosulfur compounds, onto the edge of a phyllosilicate. The invention also provides a method for producing composite materials from the edge modified phyllosilicates which includes the step of dispersing the edge modified phyllosilicate in a polymer. The invention also provides organophyllosilicates and composite materials made from the edge modified phyllosilicates.
In addition to the use of edge modification, the present invention overcomes the problems associated with the design and production of highly dispersible organoclays through the use of surface HLB modifying agents and polymeric hydrotropes which are capable of producing enhanced swelling capabilities in nonpolar systems at relatively low polymer loadings. More particularly, the present method provides organoclays which have a wide variety of uses including uses as water treatment and rheological control agents, but are particularly suited to the preparation of polyolefin-based composite materials. The present method is particularly valuable because it produces a self-activated organoclay having an expanded basal spacing with only a minor increase in organic loading.
One aspect of the present invention provides an edge modified organophyllosilicate made from a phyllosilicate having an edge modifying surfactant adsorbed along its edges to render the phyllosilcate platelet edges organophilic (i.e. hydrophobic). This can be accomplished by dispersing a phyllosilicate in a suitable solvent, such as water, along with an anionic surfactant which adsorbs preferentially to the edges of the phyllosilicate. In one embodiment of the invention the surface of the phyllosilicate is substantially free of surface HLB modifying agents. In this embodiment the phyllosilicate may be either a phyllosilicate that is able to undergo ion-exchange (hereinafter an xe2x80x9cion exchangeable phyllosilicatexe2x80x9d), such as a smectite clay or mica, or a phyllosilicate that is substantially unable to undergo ion-exchange (hereinafter a xe2x80x9cnon ion exchangeable phyllosilicatexe2x80x9d), such as a kaolinite clay or talc or smectite clays with naturally low exchange capacities. As used herein, the phrase substantially non ion exchangeable means a clay having an ion exchange capacity of less than about 70 milliequivalents per 100 grams of clay.
In another embodiment, the present method involves adsorbing a surface HLB modifying agent and, optionally, a polymeric hydrotrope onto the surface of a naturally hydrophilic phyllosilicate, such as an ion exchangeable phyllosilicate, to render the surface of the phyllosilicate more hydrophobic. For phyllosilicates that contain exchangeable cations, the basal surface of the phyllosilicate can be modified by cation exchange in the presence of a nonionic polymeric hydrotrope. In this embodiment, the edge treatment may be performed before or after the basal surface modification, but is more preferably carried out prior to basal surface modification. The polymeric hydrotrope can be adsorbed in an amount from about 0.1 weight percent to about 10 weight percent relative to the weight of the clay. Once the edge- and basal surface-modified clays have been formed in dispersion, the resulting hydrophobic organoclay can be separated by filtration, washed with water to remove excess salt resulting from the cation exchange, and dried to a desired solvent content. The organoclay can then be dispersed into a compatible solvent including desired organic solvents or used in the preparation of composite materials.
Suitable ion exchangeable phyllosilicates for use in the present invention include the smectite clays, which may have exchange capacities of at least 75 milliequivalents per 100 g of clay. Another ion exchangeable clay that is suitable for use with the invention is mica. Non-limiting examples of smectite clays include hectorite, montmorillonite, beidelite, stevensite, nontronite, and saponite. Synthetic clays are also acceptable. Suitable non ion exchangeable phyllosilicates for use in the present invention include kaolinite clays and talc. Since the kaolins and talcs do not possess a significant number of ion exchange sites on their basal surfaces, these minerals are suitably modified for composite material applications by treatment of the platelet edges with anionic surfactants without treatment of the basal surfaces via cation exchange and hydrotrope adsorption.
The present organically modified phyllosilicates can be used in many applications where phyllosilicates, and in particular hydrophobic phyllosilicates, are desired. Such applications include use as solvent thickeners, gelling agents and the like.
The above described embodiments are set forth in more detail in the following description and illustrated in the drawings described hereinbelow.